Silicon-based ceramic fibers are used as reinforcement materials in composite applications, such as metal matrix composites and ceramic matrix composites (CMC's). At high temperatures, crystalline silicon carbide (SiC) fibers have superior thermal stability and chemical resistance over silicon oxycarbide fibers, such as Nicalon.TM. and Tyranno.TM. fibers. Silicon oxycarbide fibers tend to lose oxygen at temperatures above 1,200.degree. C., and the crystallization of SiC results in formation of a porous structure. As a result, the fiber loses nearly all of its mechanical strength.
Several processes have been used to manufacture high strength silicon carbide (SiC) fibers. U.S. Pat. Nos. 5,279,780 and 5,366,943 to Lipowitz et al. describe the formation of crystalline SiC fibers by doping commercial Nicalon.TM. and Tyranno.TM. fibers with boron containing species followed by densification at higher temperature. These commercially available fibers are prepared from expensive polycarbosilane precursors. Use of polycarbosilanes presents several problems in production of boron silicon oxycarbide fibers. For example, the polymers are slow to cross-link and the green fibers produced are fragile. These problems make continuous processing difficult. Also, low yields and complicated processes cause the fibers to be expensive.
Japanese patent application JP-A-57-56567 by Yajima et al. discloses a heat resistant inorganic fiber and method for its manufacture. The fiber typically has 27 to 40 mol % silicon carbide, 10 to 15 mol % B.sub.4 C, and 45 to 63 mol % free carbon. The fiber is prepared by dry spinning a polymer that contains mainly Si, B, and O into a fiber, curing the fiber by heating, and pyrolyzing the fiber by heating to high temperature. However JP-A-57-56567 does not disclose the use of a polymer blend for preparation of SiOCB or SiCB fibers.
The use of siloxane polymers as precursors to crystalline SiC fibers is described in U.S. Pat. No. 5,167,881 to Atwell et al. The siloxane polymers are phenyl-containing polyorganosiloxane resins with 3 to 6 weight percent silanol groups. Boron is incorporated into the siloxane polymers either prior to or during formation of the fibers, or during at least one of the infusibilizing or pyrolyzing steps of the process to produce substantially polycrystalline SiC fibers. The boron is incorporated by exposing the precursor or fiber to an atmosphere containing a gaseous boron-containing compound such as diborane or boron trichloride.
The use of polymer blends to produce boron-containing carbonaceous fibers is disclosed in U.S. Pat. Nos. 4,931,100 and 4,832,895 to Johnson. A precarbonaceous polymer, such as polyacrylonitrile, was blended with a borane polymer, such as that formed by the reaction of a borane compound with a Lewis base. Boron-containing carbon fibers were obtained by either dry or wet spinning of such blends, followed by oxidation cure and pyrolysis. However, these patents do not disclose the use of siloxane polymers in the blend.
One object of this invention is formation of amorphous SiOCB fibers from a blend of a siloxane resin and a boron containing polymer. A further object of this invention is to provide a high strength SiOCB ceramic fiber.
Another object of this invention is formation of crystalline SiCB fibers from a blend of a siloxane resin and a boron containing polymer. A further object of this invention is to provide a high strength SiCB ceramic fiber.